Fix mtime for tar command and use numeric user/group

Summary:
Epoch values have to be preceded by '@'. Otherwise it seems that tar
sets a time stamp of -2^63, which we definitely don't want.

Since we use --numeric-owner, we might as well use 0 instead of root.

Follows the advice of https://reproducible-builds.org/docs/archives/,
this time correctly.

Reviewers: kfunk

Reviewed By: kfunk

Subscribers: kdevelop-devel

Tags: KDevelop

Differential Revision: https://phabricator.kde.org/D13832